The present invention relates generally to digital communication networks, and more specifically, to packet over optics networks.
In the early days of fiber optics, every telephone company had its own proprietary optical Time Division Multiplexing (TDM) protocol. In 1985 the telecommunications industry began working on a standard called SONET. This work resulted in a SONET standard in 1989. The advent of technologies such as Asynchronous Transfer Mode (ATM) over SONET and direct mapping of IP over SONET frames has extended the useful life of this technology. Packet over SONET has recently been used to support the deployment of IP-based video and voice applications. Packet over SONET places the IP layer directly above the SONET layer and eliminates the overhead needed to run IP over ATM over SONET (FIG. 1A). FIG. 1B illustrates conventional Ethernet over SONET. Virtually all the long distance telephone traffic in the United States and elsewhere now uses trunks running SONET in the physical layer.
SONET and SDH are a set of related standards for synchronous data transmission over fiber optic networks. SONET is short for Synchronous Optical Network and SDH is an acronym for Synchronous Digital Hierarchy. SONET is the United States version of the standard published by the American National Standards Institute (ANSI). SDH is the international version of the standard published by the International Telecommunications Union (ITU).
SONET/SDH is currently used in wide area networks (WAN) and metropolitan area networks (MAN). A SONET system consists of switches, multiplexers, and repeaters, all connected by fiber. The connection between a source and destination is called a path. The basic SONET frame is a block of 810 bytes transmitted every 125 μsec. Since SONET is synchronous, frames are emitted whether or not there is any useful data to send. The first three columns of each frame are reserved for system management information. The first three rows contain section overhead and the next six contain line overhead. A section is a fiber going directly from any device to any other device, with nothing in between. A line is run between two multiplexers. The section overhead is generated and checked at the start and end of each section, whereas the line overhead is generated and checked at the start and end of each line. The first column of user data (called the synchronous payload envelope (SPE)) is the path overhead (i.e., header for the end-to-end path sublayer protocol). The section, line, and path overheads contain a profusion of bytes used for operations, administration, maintenance, and provisioning (OAM&P). Since each byte occurs 8000 times per second, it represents a PCM (pulse code modulation) channel. Three of these are used to provide voice channels for section, line, and path maintenance personnel. Other bytes are used for framing, parity, error monitoring, IDs, clocking, synchronization, and other functions.
SONET/SDH and optical fiber have emerged as significant technologies for building large scale, high speed, Internet Protocol (IP) based networks. However, it is desired to eliminate the intervening SONET/SDH layer in future packet over optics networks for cost efficiency, ease of management, and bandwidth efficiency.
One standard that describes the interconnection of computer devices in local area network (LAN) communication is IEEE (Institute of Electrical and Electronic Engineers) standard 802.3, commonly referred to as Ethernet (also adopted as International Standard ISO/IEC 8802-3). The Ethernet system includes an Ethernet frame that consists of a standardized set of bits used to carry data over the system. The fields of an Ethernet packet (also referred to as an Ethernet frame) include address fields, a variable size data field that carries from 46 to 1500 bytes of data, and an error checking field that checks the integrity of bits in the frame to make sure that the frame has arrived intact. The Ethernet frame encapsulates payload data by adding a 14 byte header before the data and appending a 4-byte (32-bit) cyclic redundancy check (CRC) after the data. The entire frame is preceded by a small idle period (the minimum inter-frame gap, 9.6 microseconds) and an 8 byte preamble.
In the case of 10 Mbps and 100 Mbps Ethernet, the preamble is typically used to allow time for the receiver in each node to achieve lock of the receiver Digital Phase Lock Loop which is used to synchronize the receive data clock to the transmit data clock. At the point when the first bit of the preamble is received, each receiver may be in an arbitrary state (i.e., have an arbitrary phase for its lock clock). During the course of the preamble it learns the correct phase, but in doing so, it may miss (or gain) a number of bits. A special pattern, known as the start of frame delimiter, is used in conventional systems to mark the last two bits of the preamble. When this is received, the Ethernet receive interface starts collecting bits for processing by the MAC (medium access control) layer.
For 1 Gbps Ethernet (IEEE 802.3a), 8b/10b transmission code is used. Synchronization and code group alignment makes use of the comma control character. The preamble (including the start of frame field) has a length of eight bytes and has been retained only for backward compatibility. The preamble (excluding the start of frame field) is thus, for the most part, extra overhead which is not fully utilized.
Ethernet is typically not used in WAN applications since it does not provide operations, administration, maintenance, and provisioning capabilities. It would be desirable to modify the standard Ethernet packet to provide OAM&P capabilities and eliminate the need for the SONET/SDH layer in packet over optics networks.